The invention relates to a device for optical heterodyne or homodyne detection of an optical signal beam, which device comprises a local oscillator, an optical system comprising a polarization-sensitive beam-splitting layer for splitting the signal beam into two orthogonally polarized sub-beams and a beam-combining layer for combining signal beam radiation with local oscillator radiation, said device further comprising a detection system for converting the combined radiation into at least one electric signal which is suitable for further processing.
Devices for optical heterodyne detection are used in optical signal transmission. By mixing the signal beam in a heterodyne detection device with an optical beam from a local oscillator, a considerably better result with regard to the signal-to-noise ratio and the discrimination of background radiation is obtained as compared with direct detection of the signal beam.
The principle of heterodyne detection of optical radiation has been extensively described in the article "Optical Heterodyne Detection" by O. E. DeLange in the journal "IEEE Spectrum" of October 1968, pp. 77-85. As has been stated in this article, it is important that the states of polarization of the signal beam and the local oscillator beam correspond as much as possible. A possible solution to achieve this is to split the signal beam into two sub-beams having a mutually orthogonal state of polarization. The two sub-beams are then combined with local oscillator radiation which is polarized in the same state.
In principle, four components are required for splitting and combining the beams: two polarization-sensitive beam splitters for splitting the signal beam and the local oscillator beam, respectively, and two beam-combining elements for combining the sub-beams formed. By firstly combining the signal beam with the local oscillator beam and only thereafter splitting it by means of a polarization-sensitive beam splitter into two orthogonally polarized sub-beams, one of the beam-combining elements can be dispensed with. Since a beam-combining element not only has two inputs but necessarily also two outputs, two polarization-sensitive beam splitters remain required to receive and detect all signal radiation.
A device as described in the opening paragraph is known from EP-A 0,345,889, which corresponds to U.S. Pat. No. 5,003,625 see FIG. 3 of said document. In this device the beam-splitting and beam-combining elements are integrated in an optical component comprising two beam-splitting layers. The first layer is a polarization-sensitive beam-splitting layer, while the other beam-splitting layer is neutral with regard to the state of polarization of the incident light and is used as a combining element. However, a layer which is really polarization-sensitive is difficult to make and is thus expensive. Such a really neutral layer in the known device is required because polarization effects which would otherwise occur on this layer functioning as a beam-combining element will have a distinct effect on the operation of the coherent detection device.